Bis U Basic Manual English

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BIS U Basic Manual Basic Information for Operating a UHF RFID System English www.balluff.com BIS U Basic Manual 1 2 3 4 5 6 7 www.balluff.com Introduction4 Safety distances to the antenna 5 Physical fundamentals 7 3.1 Physics of the transmitting antenna 3.2 Physics of the transponder 8 9 Reference antennas and antenna parameters 10 4.1 Reference or standard antennas 4.2 Antenna gain 4.3 Return loss and voltage standing wave ratio 4.4 Dispersion angle 4.5 Front-to-back ratio 4.6 Impedance 4.7 Polarization 4.8 Power rating 10 11 11 12 12 13 13 14 Antenna cable 15 Calculating the radiated power 16 Component properties and system characteristics 17 7.1 7.2 7.3 7.4 7.5 7.6 17 17 18 19 19 20 Memory topology of the data carrier Structure of the EPC code Data carrier antenna shapes Directional characteristics of the data carrier dipole antenna Responsiveness of the data carrier - response field intensities Theoretical reading range 8 Reflection, dispersion and adsorption of electromagnetic waves 20 8.1 Changes in the polarizing axis ratio 8.2 Effect of different environmental conditions 8.3 Attenuation of electromagnetic radiation 22 22 23 9 Antenna and transponder mounting distances 24 9.1 Antennas on a processor 9.2 Distances to structures in the surrounding area 9.3 Mounting transponders 24 24 24 10 Operating several processors  25 10.1 Listen before talk and frequency hopping 10.2 Creating a transmission plan 25 25 11 Measures for improving the operational reliability of UHF systems  26 11.1 Field reserve and working distance 11.2 Using several antennas 26 27 3 BIS U Basic Manual 1 Introduction This document describes the physical method of operation of the RF identification system BIS U and the specifications of individual components within the overall system. The document focuses specifically on issues relating to the planning and optimization of identification systems in the following application areas: goods receipt, warehousing, production logistics, production control and distribution. Furthermore, the propagation and characteristics of electromagnetic waves in the surrounding environment and interaction with product carriers and building installations are explored in detail from a practical perspective. A separate section includes antenna safety distances for different antenna configurations, which people must be maintain if they intend to remain within the wave range of antennas temporarily or permanently. Performance characteristics such as working frequencies and radiated power specified in this documentation apply to countries within the European Union. National regulations must be observed if system components are used outside of this region. 4 BIS U Basic Manual 2 Safety distances to the antenna When using the BIS U identification system, it is possible that people will remain within the wave range of the antennas briefly or for longer periods. In addition to product standard EN 302 208, which was designed to protect other radio services against interference or negative influences from the RFID system, the International Commission on Radiological Protection (ICRP) has defined a set of HF field limit values to avoid the damage of human tissue through HF fields. These so-called basic values represent the specific absorption SA J/kg or specific absorption rate SAR in W/kg and describe the direct or indirect impact of radiation waves on human tissue. Derived values that can be measured or calculated using simpler methods are adopted for practical applications. These were defined in such a way that the basic values are never exceeded, even under the most unfavorable exposure conditions. Always observe human exposure regulations EN50364 when operating the device with a connected antenna. The applicable limit values are also outlined in the following EU Directives: –– EU Directive 1999-519 Public, –– EU Directive 2004-40 Employees. The following limit values therefore apply to a working frequency of 868 MHz: Electric field intensity: E = 40.51 V/m Magnetic field intensity: H = 0.109 A/m HF power density: S = ExH = 4.42 W/m2 Table 1: Limit values For a conventional long-range antenna with a radiated power of 2 wattERP in the main dispersion direction, this lower limit value is usually exceeded if the distance is greater than 24 cm. The safety distance decreases accordingly for lower transmission powers. This occupational safety regulation stipulates that people should not remain closer than 24 cm to the antenna for longer periods. www.balluff.com 5 BIS U Basic Manual 2 Safety distances to the antenna The following measures can be taken to meet the occupational safety regulation: –– Organizational measures that require the preparation of operating instructions containing relevant information to ensure safe operation and that draws attention to the possibility of exposure to electromagnetic fields. –– Secure the antennas by mounting protective equipment or setting up cordons to make sure people cannot remain too close to the antenna during operation. An inspection should always be carried out at appropriate time intervals before and after the task is performed. According to what is currently known, a brief stay in the vicinity of antennas does not pose a health risk. Under certain circumstances during operation, the reader and antenna may interfere with pacemakers while the pacemaker wearer is within range of the antenna. If in any doubt, the person involved should contact the pacemaker manufacturer or their doctor. 100 90 Exposure range 1 BGR B11 EU Directive 2004-40 Employ. 80 El. field intensity in V/m 70 60 Exposure range 2 BGR B11 26. BlmSchV EU Directive 1999-519 Pop. 50 40 Field intensity curve 30 20 10 0 0 20 40 60 80 100 120 Distance to the antenna in cm Fig. 1: Electric field in the vicinity of the antenna for 2 wattERP. Both components of the circular polarized antenna taken into consideration 6 BIS U Basic Manual 3 Physical fundamentals The BIS U system belongs to the class of UHF identification systems. Data carriers with air interface protocol structured according to ISO 18000-6C or the EPCglobalTM Class 1 Generation 2 standard are supported. Performance characteristics such as working frequencies and permitted radiated power are defined by European standard EN 302 208 V1.2.1: UHF band: 865 MHz … 868 MHz Radiated antenna power: max. 2 wattERP Channel bandwidth 200 kHz Channel spacing 600 kHz Channel configuration Channel 4: 865.7 MHz Channel 7: 866.3 MHz Channel 10: 866.9 MHz Channel 13: 867.5 MHz Table 2: Performance characteristics The UHF technology used here facilitates a communication distance of several meters, even for passive transponders. www.balluff.com 7 BIS U Basic Manual 3 Physical fundamentals 3.1 Physics of the transmitting antenna The UHF antenna is an open oscillating circuit with electric fields that extend into the surrounding environment. The simplest form of UHF antenna is an electric dipole. However, field displacement occurs in the vicinity of the antenna due to the high excitation frequency. The energy accumulated in the field moves away from the antenna almost at the speed of light. Fig. 2: Schematic diagram of the displacement process The energy propagates over an ever-increasing area as it moves away from the antenna and as a result, the field intensity decreases reciprocally in relation to the distance. This process of attenuation is also known as "free space loss". 8 BIS U Basic Manual 3 Physical fundamentals 3.2 Physics of the transponder Due to their shape and size, the antennas on the data carriers are capable both of reflecting and absorbing electromagnetic waves transmitted by the BIS U identification system. The passive transponder (data carrier) does not have its own power supply (e.g. battery) and must therefore draw the energy it needs to operate from the electromagnetic field. One small portion of the HF voltage present at the antenna connections is commutated and used to supply the IC. However, the much larger portion of dispersed power is reflected. A time-controlled change in the reflection characteristics of the dipole antenna generates a backscattered electromagnetic wave with a modulated amplitude (intensity). The wave is detected by the antenna on the processor and then demodulated. This type of information exchange between the partners of an identification system is known as electromagnetic backscatter. Processor Transmitting/receiving antenna Data carrier Dipole antenna transponder Directional coupler Emitted wave Emitter/ receiver Reflected wave Load resistance Free space wave Z0 Fig. 3: Schematic diagram of backscatter www.balluff.com 9 BIS U Basic Manual 4 Reference antennas and antenna parameters Only passive antennas can be connected to the BIS U processor. The power of the antennas is defined by a generally binding parameter set of measurable properties which include: –– –– –– –– –– –– –– –– 4.1 Reference or standard antennas Antenna gain, Polarization, Axial ratio, Dispersion angle, Front-to-back ratio Return loss / VSWR, Impedance, Power rating. The comparability of different antennas and the quantitative assessment of power radiated by the antennas are achieved using reference or standard antennas. The following antennas are used for reference purposes: Isotropic radiator The isotropic radiator is a hypothetical, lossless antenna that disperses evenly in all directions and generates a power density independent of the angle at distance r. Half-wave dipole (λ/2 dipole) The maximum field intensities are vertical to the dipole level. A power density is generated in the shape of a figure eight. If the same high-frequency power is supplied to both antennas, the half-wave dipole has a higher field intensity than the isotropic radiator in the main dispersion directions. There is a direct correlation between the two values: the radiated power of a half-wave dipole in the main dispersion direction is higher than that of the isotropic radiator by a factor of 1.64 (2.15 dB). 180 150 210 120 240 Isotropic radiator 90 270 Half-wave dipole 60 300 30 330 0 Fig. 4: Vertical radiation diagram for reference antennas 10 BIS U Basic Manual 4 Reference antennas and antenna parameters 4.2 Antenna gain Real antennas bundle radiation and therefore generate maximum radiated power density in one direction (main dispersion direction). In order to make antennas with different designs or directional characteristics comparable and define a dimension to indicate the intensity of radiated antenna power aimed in a preferred direction, antenna gain must be used. The antenna gain represents the factor by which power radiated in the main dispersion direction is higher than a reference antenna. Indicating the gain of a real antenna in relation to the isotropic radiator is commonplace. G[dBi] Linear gain based on the isotropic radiator G[dBic] Circular gain based on the isotropic radiator Figure 3 shows that radiation is also bundled for the half-wave dipole. The antenna gain based on the isotropic radiator is: G[dBi] half-wave dipole = 2.15 dBi 4.3 Return loss and voltage standing wave ratio www.balluff.com The voltage standing wave ratio (VSWR) and the return loss (RL) indicate how much of the energy flowing through the cable to the antenna is reflected to the receiving antenna input on the processor. A poor VSWR value can cause interference or noise. A typical value < 1.2 to 1 is specified for the BIS U 300 antenna. 11 BIS U Basic Manual 4 Reference antennas and antenna parameters 4.4 Dispersion angle A specified dispersion angle provides another parameter indicating the directional characteristics of an antenna. The identified dispersion angle is the angle at which only half the power is radiated, which represents a decrease in power of 3 dB. The reference variable is the maximum value in the main dispersion direction. Since antennas are always passive components, the antenna gain correlates directly with the dispersion angle: the higher the antenna gain, the smaller the dispersion angle. In valid product standard EN 302 308 (issue V1.1.2 2006-07), the permitted radiated power of an antenna correlates with the antenna dispersion angle as follows: –– Dispersion angle –– Dispersion angle ≤ 70 degrees > 70 degrees Radiated power Radiated power up to 2 wattsERP up to 0.5 wattsERP 3 dB dispersion angle 0 30 0 -3 330 -6 -9 60 300 -12 -15 -18 -21 -24 90 270 120 240 150 210 180 Front-to-back ratio Fig. 5: Radiation diagram of a real antenna - horizontal section Two dispersion angles are specified to provide a full description: the vertical dispersion angle (elevation) and the horizontal dispersion angle (azimuth). 4.5 Front-to-back ratio The electromagnetic waves are also radiated by directional antennas, not only in the main dispersion direction, but also in other spatial directions, in particular a backwards spatial direction. These minor lobes should be suppressed as efficiently as possible to allow the radio fields to be aligned correctly towards the selected data carrier. Attenuation in a backwards dispersion direction in relation to power radiated in the main dispersion direction is described as the front-to-back ratio (see figure 4). A typical value > 18 dB is specified for the BIS U 300 antenna. 12 BIS U Basic Manual 4 Reference antennas and antenna parameters 4.6 Impedance All components must have the same real impedance to allow the transfer of power between the processor and the antenna. The BIS U system is designed for connecting system components (antenna and cable) with a wave resistance or impedance of Z = 50 Ω. Deviations in the impedance will result in maladjustment that may cause reflections or vertical waves and significantly reduce the performance of the overall system. 4.7 Polarization The field vector of electromagnetic waves into the surrounding environment is directional. The alignment of the field vector or the directing of vibrations is described as wave polarization. A distinction is made between linear and circular polarization, whereby antennas with the latter characteristic are more important because the field intensity value for circular polarized waves is the same regardless of the spatial orientation. The reception characteristics of most UHF data carriers are similar to those of a dipole antenna due to their design. Transmitting antennas with circular polarization are used to ensure that the data carriers function correctly whatever their position. Fig. 6: Circular polarized wave With circular polarization, an additional distinction is made between circular polarization in a counterclockwise and clockwise rotational direction. But these characteristics are irrelevant in practical applications because transponders usually have the same properties as linear polarized antennas. www.balluff.com 13 BIS U Basic Manual 4 Reference antennas and antenna parameters On real antennas, however, the displacement achieved along both spatial axes is never exactly the same. The polarization ellipse that develops is illustrated by the axial ratio of the two components. A typical value 1 dB is specified for the BIS U 300 antenna. (V axis) / (H axis) = 2 or 3 dB (V axis) / (H axis) = 1 or 0 dB Vertical axis (V axis) Horizontal axis (H axis) Fig. 7: Axial ratio of a circular polarized antenna 4.8 Power rating 14 Describes the maximum effective power with which the antenna can operate. BIS U Basic Manual 5 Antenna cable Only coaxial antenna cables with a wave resistance or impedance of Z = 50 Ω may be used to prevent reflections and vertical waves (resonances) in the antenna line. Losses resulting from the transfer of electric power to the antenna are known as cable attenuation. The degree of the cable attenuation depends entirely on the length of the cable, which is selected based on the cable diameter, cable configuration and frequency response. As a general rule, the cable manufacturer specifies the cable attenuation in dB per meter (dB/m). www.balluff.com 15 BIS U Basic Manual 6 Calculating the radiated power The measurable value in the main dispersion direction always defines the radiated antenna power. Within the jurisdiction of the EU, limit values relating to the power radiated from antennas are calculated using a so-called Effective Radiated Power (ERP) based on the half-wave dipole. Therefore, the ERP value describes the effective power that a dipole antenna supplied with P0 radiates in the preferred direction. The ERP value of an antenna whose gain is defined based on the isotropic radiator is calculated according to the following equation: with P0[dBm] Antenna supply power ERP = P0 + Gi – 2.15dBi Gi[dBi] Antenna gain based on isotropic radiator 2.15 dBi Gain of dipole based on isotropic radiator The equations for calculating the radiated power of antennas are logarithmic and the power data is standardized to 1 mW because addition is simpler to perform. As a result, all required antenna and power parameters can be specified in decibels and simply added to one another. The following parameters are required or used to calculate the radiated antenna power: Socket output power of the processor based on P0[dBm] 1 mW G[dBic] Circular antenna gain based on the isotropic radiator G[dBi] half-wave dipole = 2.15 dB Antenna gain of half-wave dipole Ak[dB] Cable attenuation per meter L[m] Cable length in meters The following formula can therefore be used to calculate the equivalent radiated power of an antenna based on the half-wave dipole: (1) ERP[dBm] = P0[dBm] – Ak[dB] • L[m] + G[dBic] – 2.15 dB The formula can be rearranged to calculate the permitted socket output of the processor: (2) P0[dBm] = ERP[dBm] + Ak[dB] • L[m] - G[dBic] + 2.15 dB POWER Watts dBm 2.000 33 1.000 30 0.500 27 0.250 24 0.125 21 Table 3: Correlation table 16 BIS U Basic Manual 7 Component properties and system characteristics In order to make the selection of system components easier and ensure that they perform the relevant tasks in the application correctly, this section discusses some of the basic properties and characteristics of UHF components. 7.1 Memory topology of the data carrier 7.2 Structure of the EPC code The memory of a UHF data carrier is divided up as follows: 96 bit EPC read/write memory (option of extending to 256 bit). 512 bit Freely accessible read/write memory area for customer-specific applications. 32bit+64 bit TID - Fixed unique product and serial number 32 bit Access password 32 bit Kill password for destroying the transponder (not supported) EPC codes were introduced to provide a migration path for the transition from the barcode to RFID technology. ELECTRONIC PRODUCT CODE 0 1 . 0 0 0 0 A 8 9 . 0 0 0 1 6 F. 0 0 0 1 6 9 D C 0 Header 0-7 bits EPC Manager 8-35 bits Object Class 36-59 bits Serial Number 60-95 bits Fig. 8: EPC code Header bit positions (0 .. 7) represents the length of the EPC code, possible lengths from 64 to 256 bits, 01 length 96 bit EPC manager bit positions (8 .. 35) describes the product manufacturer Object class bit positions (36 .. 59) describes the product (stockkeeping unit) Serial number bit positions (60 .. 96) used to make a distinction between 2 37 individuals www.balluff.com 17 BIS U Basic Manual 7 Component properties and system characteristics 7.3 Data carrier antenna shapes Antenna designs are predominantly limited to shapes that are very similar to a dipole because the power is drawn exclusively from the electric field in the far field. Data carriers that incorporate slot, patch or microstrip resonator antennas are exceptions to the rule because they can be mounted directly onto metal surfaces. This document does not explore these data carriers in further detail. Data carriers with antennas similar to a dipole are available in a wide range of shapes and sizes, for example: Source: Alien Technologies Source: UPM Raflatac Source: Alien Technologies Fig. 9: Data carriers with different antenna designs The use of RF loop antennas with additional radiating elements (wave trap dipoles) achieve a reduction in the overall size and make data carriers suitable for use in the near field. 18 BIS U Basic Manual 7 Component properties and system characteristics 7.4 Directional characteristics of the data carrier dipole antenna The data carrier is sensitive to orientation because of the dipole antenna principle. Antenna 0 330 30 60 300 90 270 v 240 x φ 120 210 150 180 α z 0° position represents the flat position on the x-y plane. Fig. 10: Directional sensitivity of the data carrier dipole antenna The following qualitative statements apply: –– If a circular polarized transmitting antenna is used, no directional sensitivity is observed when the antenna is rotated around the z-axis. –– When the antenna is rotated around the x-axis, a reduction in sensitivity is observed at rotation angles 90° and 270°. –– When the antenna is rotated around the y-axis, read capability is absent at rotation angles 90° and 270°. 7.5 Responsiveness of the data carrier - response field intensities In order to supply power to the ICs, passive RFID data carriers are instructed to draw the required energy from electromagnetic waves radiated by the antenna on the processor. The response field intensity is the minimum field intensity required to operate the integrated circuit that is present at the location of a data carrier. Use of the external electric field, which generates a sufficiently high HF voltage at the antenna connections, is largely influenced by the antenna design and capacity to adjust to the working frequency. The power consumption of the data carrier ICs varies for each individual semiconductor manufacturer and design generation, and has an influence on the responsiveness of the data carrier. Responsiveness is a particularly important aspect because the processor can usually detect a data carrier located in a HF field of sufficient intensity. www.balluff.com 19 BIS U Basic Manual 8 Reflection, dispersion and adsorption of electromagnetic waves 7.6 Theoretical reading range Under ideal conditions, the electric field intensity in the near field (approximately > 70 cm) decreases reciprocally in relation to the distance (free space loss). Varying the antenna power generates an array of curves that allocates a unique field intensity to every point within the surrounding environment. The theoretical reading range for the different levels of power radiated from the antenna can be determined by dissecting the response field intensity with the relevant field intensity curve. Under ideal boundary conditions, this type of reading range value is calculated in a free field or a large absorber chamber. Antenna power in watts 2.0 1.0 0.25 Transponder I 260 cm 190 cm 90 cm Transponder II 820 cm 560 cm 280 cm Table 4: Theoretical reading ranges as a function of the antenna power 8,00 7,00 El. field intensity in V/m 6,00 5,00 4,00 3,00 2,00 1,00 0,00 0 100 200 300 400 500 600 700 800 900 1000 Distance to the antenna in cm Fig. 11: Determining the theoretical reading range Electromagnetic waves radiated from the antenna propagate almost at the speed of light and meet objects with different consistencies. The wave can be absorbed and reflected or scattered in all directions at different intensities. 20 BIS U Basic Manual 8 Reflection, dispersion and adsorption of electromagnetic waves Apart from the consistency of the material, which may be similar to metal or polar liquids, the size of the obstacles has a decisive influence on the backscattering characteristics: Rayleigh range The reflections are negligible if their dimensions are much smaller than the wave length. Resonance range The object size is comparable with the wave length. Resonant absorption and radiation from sharp objects, slots and points are observed and may cause changes in the polarization direction or result in the magnification or obliteration of fields. Optical range The object dimensions are large compared to the wave length. The geometry and position of the object (incidence angle of the wave) have an influence on the backscattering result. Experiences gained from the field of geometrical optics can be used in an approximate capacity. Parts of the primary wave that overlap with stray partial waves generated by reflections, scattering or diffraction on metallic structures in the actual surrounding area can result in local magnification or reduction in the electric field intensity. If the field intensity decreases so much that it falls below the response field intensity value for the data carrier, communication between the processor and data carrier is interrupted. However if interaction in the surrounding area at a point situated further in front of the antenna causes the field intensity to increase, communication between the data carrier and processor remains stable. Field magnification can result in superrefraction as a result. For this reason, it is not possible to specify a reading range for a specific UHF identification system consisting of a data carrier and antenna/processor that is valid for all applications or boundary conditions. 8,00 7,00 Anticipated field intensity curve according to theoretical free space loss El. field intensity in V/m 6,00 Field intensity curve measured 5,00 4,00 3,00 Data carrier response field intensity 2,00 1,00 Area without communication 0,00 0 50 100 150 200 250 300 350 400 450 500 Distance to the antenna in cm Fig. 12: Appearance of areas without communication (blind spots) As already highlighted, interaction between electromagnetic waves and objects in the actual surrounding area as well as between the waves themselves results in changes in the anticipated electric field distribution or free space loss. Selected examples should be used to demonstrate the effect of this interaction on the performance of UHF technology. www.balluff.com 21 BIS U Basic Manual 8 Reflection, dispersion and adsorption of electromagnetic waves 8.1 Changes in the polarizing axial ratio Interaction with structures in the surrounding area changes the axis components of a circular polarized wave, which are originally almost identical in size. These changes result in noticeable differences in the reading performance or reading range depending on the degree of response field intensity and on whether the data carrier is mounted in a vertical or horizontal position. 8 7 El. field intensity in V/m 6 5 Vertical component 4 3 2 Horizontal component 1 0 0 50 100 150 200 250 300 350 400 450 Distance to the antenna in cm Fig. 13: Changes in the axial ratio of polarization components 8.2 Effect of different environmental conditions The positioning, materials and geometry of the obstacles in the surrounding environment can vary from application to application and can therefore be expected to have a direct effect on the appearance of the electric field distribution. In diagram 13, the vertical component of the electrical field intensity curve has been measured comparatively for three different spaces in the main propagation direction using a circular polarized antenna. 8,00 Free space loss - vertical component 7,00 ______ Space I El. field intensity in V/m 6,00 ______ Space II ______ Space III 5,00 4,00 3,00 2,00 1,00 0,00 0 100 200 300 400 500 Distance to the antenna in cm Fig. 14: Free space loss curve for three different spaces in main dispersion direction 22 600 700 BIS U Basic Manual 8 Reflection, dispersion and adsorption of electromagnetic waves 8.3 Attenuation of electromagnetic radiation It is well-known from low-frequency RFID systems that waves permeate all electrically non-conductive materials virtually without loss. On UHF systems, a different approach must be adopted when assessing the behavior of waves penetrating materials. –– Solids or liquids that are comprised of polar molecules and contain water or carbonic substances, for example, present a high degree of HF attenuation and weaken the radiation emitted by the antenna significantly. This information confirms that the human body represents an insurmountable obstacle for the propagation of electromagnetic waves. –– Mineral oils on the other hand only weaken electromagnetic waves to an extremely limited extent because they consist of non-polar molecules. As a result, SmartLabels can be affixed directly to plastic mineral oil containers, for example. –– UHF waves cannot penetrate metallic surfaces or grid structures consisting of metallic rods or mesh. This group also includes metal reinforced concrete walls. –– Electrically non-conductive, dry materials such as plastic, paper and wood are penetrated virtually without loss. www.balluff.com 23 BIS U Basic Manual 9 Antenna and transponder mounting distances 9.1 Antennas on a processor Even if the antennas are connected to a processor, the minimum distances for the following configurations should be respected to prevent unwanted interaction: –– Two antennas installed beside one another –– Two antennas installed back to back > 50 cm > 50 cm 9.2 Distances to structures in the surrounding area A minimum distance of 50 cm from metallic components or polar liquids must be maintained to prevent the antenna from detuning and avoid backscatter from strong electromagnetic fields. 9.3 Mounting transponders Mounting data carriers directly to metal surfaces can drastically reduce the reading range. Distances of a minimum of 15 mm from the metal surface can improve the reading performance considerably, depending on the antenna design. To prevent the data carriers from detuning, the minimum distance between two data carriers must not exceed 50 mm. 24 BIS U Basic Manual 10 Operating several processors Due to the large range available for UHF fields, it is possible that processors will have a negative influence on one another if they are operated simultaneously and randomly select the same working frequency. 10.1 Listen before talk and frequency hopping One way of avoiding interference is to configure the processors to switch to a different transmitting channel in a random or preprogrammed sequence (frequency hopping). Since the ETSI standard only allows the selection of 4 or 10 different frequency channels, the probability of two carriers simultaneously occupying the same channel is quite high. Therefore prior to transmission, the processor checks whether the channel in question is already occupied to avoid dual occupancy. The processor only starts transmitting if the selected channel is available (listen before talk) to prevent overlapping or collisions. In order to guarantee dynamic use of the transmitting channels, the transmission time may not exceed 4 seconds. The processor must then wait 100 ms or switch directly to a new unused channel. Following the amendment of ETSI standard EN 302 208 (V1.2.1), the procedure for reducing mutual interference is no longer mandatory. The procedure mentioned in section 10.2 is preferred. 10.2 Creating a transmission plan Increasing the channel spacing (ETSI 302 208 V1.2.1) to 600 kHz already rules out the mutual overlapping of transmitted and received signals traveling between adjacent transmitting channels for all available radio profiles – the position of the side band in relation to the transmission frequency is determined here. Mutual interference is prevented by allocating different transmitting channels to the processors involved, i.e. creating a transmission plan. The transmitting channels available for selection are listed in section 3. The distance to the side band can be as much as 320 kHz in Dense Reader Mode (according to ETSI TS 102 562) and allows more than one reader to be operated on each channel. www.balluff.com 25 BIS U Basic Manual 11 Measures for improving the operational reliability of UHF systems In a real environment, the primary wave emitted by the antenna reflects against large objects such as walls, floors, deposited transport containers etc. and causes independent, uncontrollable propagation in the form of a stray secondary wave. In the worst case scenario, interference between the primary wave and the secondary waves can cause field attenuation. In a multi-reflective environment, it is virtually impossible to predict the field intensity at a specific location. It should also be noted that a change in the surrounding area caused by moving transportation equipment, for example, may cause the field intensity to change over time. 11.1 Field reserve and working distance Local or temporary decreases in the field intensity have the same effect as a deliberate reduction in the transmission power during reading operations. If the transmission power of the antenna is then reduced to a point where the data carrier can just about be detected, a decrease in the field intensity within the multi-reflective environment is followed by an interruption in communication. Figures 11, 12 and 13 clearly show that the fluctuations increase in line with the distance to the transmitting antenna. In order to ensure that the electric field never falls below the excitation field intensity of the data carrier in a multi-reflective environment, even when the field intensity fluctuates, field reserves within range of the fluctuation amplitude must be taken into consideration. This results in a calculative increase in the response field intensity and definition of the so-called working distance at the intersection point with the power curve. 8,00 2 watts 7,00 El. field intensity in V/m 6,00 Theoretical reading range Working distance Field reserve 5,00 4,00 700 cm 350 cm 6 dB 0.5 watts 3,00 2,00 Field reserve 1,00 0,00 0 100 200 300 400 500 600 700 800 900 1000 Distance to the antenna in cm Fig. 15: Graphic derivation of the working distance for a data carrier When designing a UHF system, it is recommended that the following rules are observed: –– The values for the working distance of a data carrier specified by the manufacturer or system supplier should not be exceeded. –– For a data carrier positioned at the point of operation, the response field intensity must be calculated by reducing the transmission power. The transmission power must then be increased by the field reserve prior to operation. A field reserve of 50% to 100% is usually considered sufficient for most applications. 26 BIS U Basic Manual 11 Measures for improving the operational reliability of UHF systems 11.2 Using several antennas Each antenna generates a different spatial field distribution pattern because the obstacles in a multi-reflective environment are positioned differently for each antenna. It can therefore be expected that the value for the local field intensity at the transponder position will change as soon as an antenna in a different spatial position starts transmitting. Antenna 3 Antenna 2 Antenna 4 Antenna 1 Fig. 16: Arrangement of several antennas Randomly positioned data carriers can also be detected in this kind of antenna configuration. These antenna configurations can be used in the following scenarios: Incoming/outgoing Pallets containing goods are transported through the doors of a waregoods house, trading house or industrial enterprise. In the corresponding stationary antenna configuration (gantry arrangement), individual UHF data carriers affixed to pallets or even goods can be detected automatically. The scanned data may contain information about the origin and nature of the products. Internal company commodity flows Gantries with antennas are installed at selected points within the industrial company. Containers bearing data carriers are detected when they pass through the gantries. An overall picture of the flow of products throughout the production sequence can be obtained by analyzing the scanned data. Non-orientated data carriers Affixing several data carriers to rotationally symmetrical goods or containers, for example, is not viable because of data consistency issues and the costs involved. The only way to reliably detect unclearly aligned data carriers is through the use of antennas aligned towards the unidentified product from different angular positions. www.balluff.com 27 Balluff GmbH Schurwaldstraße 9 73765 Neuhausen a.d.F. Germany Phone. +49 7158 173-0 Fax +49 7158 5010 [email protected] www.balluff.com No. 883387 E • Edition 1111 • Subject to modification. www.balluff.com